1. Field of the Invention
The present invention relates to an improved method for controlling the bar code reading in a bar code reader.
2. Description of the Prior Art
In a conventional method for controlling bar code reading, a signal for controlling the turn-on of a light emitting diode (LED) which projects a light onto a bar code surface and a shift pulse signal for driving a charge coupled device (CCD) which receives reflected light from the bar code surface and reads information represented by the bar code are delivered simultaneously to provide a correspondence between the LED-on time for which the bar code is read and the time (CCD photo-detection time) during which the CCD receives the reflected light from the bar code surface, as shown in FIG. 5. Also, the time during which the bar code information read for the CCD photo-detection time (LED-on time) is delivered as an analog signal is so controlled as to be the same as the time for which the bar code information is supplied to the CCD.
A circuit which generates the above-mentioned two control signals is shown in FIG. 6.
As seen in FIG. 6, a counter 101 provided with a counter reset control unit 102 is supplied with a clock signal and an output signal divided in frequency in the counter 101 is delivered to the counter reset control unit 102 and two monostable multivibrators 103 and 104.
The above-mentioned monostable multivibrator 103 delivers a square-wave shift pulse signal which drives the CCD (namely, CCD shift pulse signal). This square wave signal is supplied via flip-flop 105 to one of the input terminals of an AND circuit 106. A square-wave signal delivered from the above-mentioned monostable multivibrator 104 is supplied to the other input terminal of the AND circuit 106 where it is ANDed with the output signal from the flip-flop 105 to produce a control signal. This control signal, that is, a logical product, is delivered from the AND circuit 106 to drive the LED.
For the correct reading of bar code information marked on the surface of a commodity, it is necessary to lengthen the LED-on time (CCD photo-detection time) to some extent.
However, the lengthening of the CCD photo-detection time will cause a possibility that the CCD is likely to be adversely affected by a disturbing light incident for this time, so that an error takes place in a bar code information read by the CCD due to the incident disturbing light or an error occurs in the bar code information supplied to the CCD due to the lateral oscillation caused during reading of the bar code, possibly delivering an incorrect information.